Elimination of virus reservoirs that persist in HIV-infected children and adults despite long-term suppression of plasma viremia by traditional antiretroviral therapy (ART) is crucial for curing HIV infection. To identify and ultimately destro these latently infected virus reservoirs that promote virus rebound, it becomes important to first specifically identify the cells and tissue sites that harbor HIV during latency induced by ART. Our long-term goal is to inhibit or prevent the development of viral reservoir host cells that aris despite ART in HIV-infected children. The purpose of this R21/R33 proposal is to verify the contributions of CD4+ T cells and macrophages as reservoir cells in SIV-infected and ART-treated pediatric macaques. Our central hypothesis is that in SIV-infected newborn macaques where pathogenesis occurs more rapidly than in adults, tissue macro- phages also serve as major virus (SIV) reservoirs after infection. The rationale for our hypothesis is that (i) SIV infections in newborn macaques parallel the rapid disease progression observed in HIV-infected children, and (ii) in the primate model, the reservoir sites of SIV can be examined in finer detail and under more controlled experimental conditions than in HIV-infected humans. We propose two aims in phase I (R21) of this application: Aim 1. To determine the contribution of CD4+ T cells to the virus reservoir in SIV-infected pediatric macaques undergoing effective ART. Our working hypothesis is that in vivo depletion of CD4+ cells in SIV- infected newborn macaques undergoing effective ART will directly demonstrate the proportion of CD4+ T cells (vs macrophages) that contribute to the SIV reservoir. We will use a well-established in vivo model via injection of anti-CD4 depleting antibody to remove CD4+ T cells in the SIV-infected macaques undergoing optimal ART. Aim 2. To determine the contribution of monocytes/macrophages to the virus reservoir in SIV-infected pediatric macaques undergoing effective ART. Our working hypothesis is that the in vivo depletion of monocyte/macrophages of the SIV-infected macaques undergoing effective ART will also demonstrate a contribution of macrophages (vs CD4+ T cells) to the SIV reservoir in the lung. We will use in vivo administration of liposome-encapsulated alendronate that temporarily depletes monocytes/macrophages in macaques. These results will ascertain if both CD4+ T cells and tissue macrophages, or either set of cells alone, develop and mature into reservoirs sites of SIV during effective ART. Tissue specimens will be examined in fine detail to measure viral levels (RNA and DNA) as well as identify specific sites and subset markers of reservoir cells infected with SIV during ART. This will set the foundation for aim 3 of phase II (R33) to corroborate if viral reservoirs were eliminated or drastically reduced by discontinuation of ART. Aim 3. To determine if in vivo depletion of CD4+ T cells and/or monocyte/macrophages prevents virus rebound after discontinuation of ART in SIV-infected newborn macaques. Our working hypothesis is that both CD4+ T cells and macrophages contribute to SIV reservoirs and that elimination of either or both cell populations followed by discontinuation of ART will lead to maintenance of low or absent viral loads. If virus rebound occurs, it will help to corroborate the remaining or alternate sites of the virus reservoirs. This study will contribute to developing rational intervention strategies to reduce or cure AIDS in HIV-infected children.